The Boeing B-9 was the first all-metal monoplane bomber used by the US Army Air Corps (USAAC). Although the B-9 was not the first USAAC monoplane bomber, this distinction goes to the Douglas B-7 and the Fokker (General Aviation) XB-8, but they were conceptually far less advanced.1

The B-9 was a privately funded Boeing project that began with the development of the single-engine Boeing Monomail commercial transport. The B-9 was basically an enlarged twin-engine version of the Monomail, using the same construction techniques. It was an innovative design that featured:

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Semi-retractable landing gear.

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Flight control servo tabs.

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Cantilever low-wing monoplane.

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All-metal, semi-monocoque fuselage with stressed skin construction.

There were two initial models of the B-9, the Model 214 (Y1B-9) and Model 215 (XB-901, YB-9). Both aircraft were identical except for the selection of engines. The Model 214 was powered by a
Curtiss V-1570 Conqueror and the Model 215 with a
Pratt & Whitney R-1860 Hornet.

The first of the two B-9 models to fly was the Model 215 on April 29, 1931 and it was during this period that the USAAC was still taking delivery of the Keystone B-6 biplane bomber. The Keystone B-6 remained the backbone of the American bomber force until 1932. In comparison, the B-9 was a radical departure in design—a low-wing monoplane with semi-retractable landing gear (the wheels were partly exposed) that took advantage of new research from the National Advisory Committee on Aeronautics (NACA).

During testing at the 20 foot NACA wind tunnel at Langley Virginia, built in 1927, engineers experimented with a Sperry Messenger and discovered that drag could be decreased by 40% with the installation of retractable landing gear. Another experiment with a
Curtiss Hawk
in 1927 showed the advantages of streamlining the engines with the NACA engine cowling. The NACA engine cowling increased airspeed from 118 to 137 mph. With the release new data, aircraft manufacturers rushed to install the NACA engine cowling and create aircraft with retractable landing gear.2

Another development that came out of Langley was the placement of the engines. If not on the nose, engines were normally placed either above or below the wing or slung between the upper-wing and lower-wing on twin-engine biplanes. Additionally, external struts supported the landing gear below the fuselage. The landing gear struts and external engine bracing created tremendous drag. NACA engineers tried different engine configurations in order to decrease drag and found it best to place the engine directly in front of the wing in a streamlined nacelle.3 This was the optimum design. This arrangement also allowed for a low-wing which permitted a retractable landing gear that would retract inside the wing. As drag was eliminated, performance increased dramatically and as new data became available, Boeing engineers created new configurations that were incorporated into the B-9. While the Keystone B-6 lumbered along at 120 mph, the new design of the Boeing B-9 increased airspeed to 163 mph—an increase of 43 mph.

The Model 215 was the first to be tested by the Army as Boeing property under the experimental military designation as the XB-901. The airplane was given the standard Army designation of B-9 and was purchased later in the year and dubbed the YB-9. The Pratt & Whitney Hornet engines used on the YB-9 gave it a top speed of 163 mph (262 km/h).

The Model 214 was the second test model, which was dubbed the Y1B-9 and was originally powered with Curtiss Conqueror engines. The increased power from these engines, combined with increased streamlining of the engine nacelles, increased its top speed to 173 mph (278 km/h). With the exception of the
Curtiss B-2 Condor, liquid-cooled engines were never used on production US military bombers, as air-cooled radial engines were lighter and more reliable than the liquid-cooled engines, and also less vulnerable to enemy damage. The Model 214 (Y1B-9) was later converted to Hornet engines.

The Model 246, designated the Y1B-9A was an improved version over the YB-9 which featured more powerful Hornet engines and a redesigned vertical stabilizer. With the new engines, the B-9 airspeed increased to 186 mph, which equaled the speed of all existing American fighter aircraft. Seeking a desirability to protect the flight crews because of the increased speed, enclosed canopies were built for the B-9 bomber, but never installed.

As in all Army bombers since World War I, the bomb bay was located inside the fuselage at the center of gravity and additional bombs could be hung from racks under the wings. The total disposable bomb load was 2,400 lbs.4

The flight controls required servo tabs to assist pilots in moving the flight controls due to higher airspeeds and greater loads on the control surfaces. This was the first time servo tabs were used on an American aircraft.5

There were five crew members. Starting from the nose their positions were:

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Bombardier.

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Radio operator.

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Pilot.

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Copilot.

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Rear dorsal gunner.

Despite the increased speed, all of the crew sat in open cockpits except for the radio operator in a station forward and below the pilot. The bombardierís nose cockpit was equipped with a bomb site and aiming window and also a flexible gun at the top. The pilots sat in tandem cockpits rather than side-by-side due to the narrowness of the fuselage. The rear gunnerís cockpit was located on top of the fuselage and aft of the wing.

Boeing always hoped that it would receive large orders for the airplane and although the B-9 was a great advance over previous USAAC bombers, only seven aircraft were built. It was quickly eclipsed by the Martin B-10 and B-12. The last B-9 was delivered March 20, 1933.